Examples of braking apparatuses for a vehicle generally include hydraulic brake equipment configured to apply a hydraulic brake. However, in electric motor vehicles or so-called hybrid motor vehicles in recent years, a regenerative brake using an electric motor is used. For example, a braking force control apparatus configured to generate allowance margins of motor torque for both the regenerating side and the power running side to enlarge a control width of the motor torque is disclosed in JP-2007-106385A (Reference 1). In JP-11-4504A (Reference 2), collaborative control of the regenerative brake and the hydraulic brake is performed. In contrast, in JP-2003-287069A (Reference 3), electric motor brake equipment configured to convert a rotary motion of an electric motor into a translatory motion to propel a piston and press a friction pad against a disk rotor to generate a braking force is disclosed. In this manner, the braking apparatus for a vehicle provided with an electric motor for driving employs also the frictional brake using a mechanical braking portion such as the hydraulic brake equipment or the electric motor brake equipment described above in addition to the regenerative brake (regenerative brake) by the electric motor.
In addition, in JP-2004-187445A (Reference 4), applying a brake by exciting the motor to rotate in the reverse direction is proposed. However, it is a general braking portion in an induction motor using no permanent magnet, and is employed in railway vehicles. In other words, in the railway vehicles (electric vehicles), since stopping is achieved only by the induction motor, a control apparatus for electric vehicles configured to apply a pure electric brake or an all-electric brake by applying an electric brake or a stop brake in addition to the regenerative brake is proposed, and braking on the basis of reverse-phase driving is employed in addition to the regenerative brake. For example, in JP-11-234804A (Reference 5), a reverse-phase electric brake configured to obtain a braking force by speed reduction and also by switching to reverse power running by a forward brake is disclosed. However, the frictional brake is still in combination use in actual railway vehicles, and, for example, an air brake system is also mounted.
As the braking apparatuses for electric motor vehicles or hybrid motor vehicles described above, a permanent magnet synchronous motor including a rotor having a permanent magnet and a stator configured to allow the rotor to be driven to rotate, and the rotor being coupled to respective wheels of the vehicle, and a power accumulating portion such as a battery configured to supply power to the permanent magnet synchronous motor to excite the stator are provided and configured to inhibit the rotation of the wheels by the permanent magnet synchronous motor and, more specifically, an embedded permanent magnet field synchronous motor (referred to as IPM) having the permanent magnet embedded into the rotor is used. Furthermore, an in-wheel motor (referred to as IWM) including the rotor and the stator of the electric motor accommodated in the wheel of the wheel is also proposed. Such an in-wheel motor, being disclosed in JP-2007-196904A (Reference 6), also employs the frictional brake in combination, and the wheel includes a hydraulic frictional brake equipment mounted thereon in addition to the electric motor.
As described above, a technology which allows the vehicle to stop only by the induction motor is focused in the railway vehicles (electric vehicles). However, in the electric motor vehicles or the hybrid motor vehicles, the frictional brake on the basis of the hydraulic fictional brake equipment is used in combination with the regenerative brake on the basis of the permanent magnet synchronous motor. Therefore, the hydraulic frictional brake equipment is essential also in the in-wheel motor in order to bring the wheels into a stop state. This may impair reduction in unsprung weight, and also impairs practical application of the in-wheel motor. In recent years, in the motor vehicles, a large cabin space such as a riding space or a luggage space is required. In the motor-driven electric motor vehicles, if a compact high-torque motor is developed in the future, reduction in diameter of the wheels is enabled, and enlargement of the cabin space is enabled correspondingly. However, as described above, since the frictional brake equipment arranged in the wheel is an essential configuration in the motor-driven vehicles in the related art, a problem that the frictional brake equipment impairs the reduction in diameter of the wheels exists.
A need thus exists for a braking apparatus for a vehicle configured to inhibit the rotation of wheels by an electric motor, in which the rotation of the wheels is inhibited only by controlling the electric motor, and a smooth and reliable braking force may be applied until the wheels are brought into a stop state without necessity of a frictional brake equipment.